At present, non-aqueous electrolyte secondary batteries such as a lithium ion battery using, as the active material, a material capable of reversibly absorbing and desorbing lithium ion have been put into practical use.
For the positive electrode of the non-aqueous electrolyte secondary battery, for example, LiCoO2, which is a lithium-containing complex oxide, is being employed. Li ions are originally contained in the positive electrode, and the Li ions are reversibly absorbed in and desorbed from a carbon material in the negative electrode during charge and discharge.
Apart from LiCoO2, lithium-containing complex oxides include LiNiO2, LiMn2O4 and complexes thereof. These oxides exhibit a potential as high as about +4 V with respect to the potential of metallic lithium and also have large reversible capacity. Therefore, they are excellent materials for use as active materials, capable of realizing batteries of high voltage and high capacity.
On the other hand, for the negative electrode of the non-aqueous electrolyte secondary battery, carbon materials are commonly used. Carbon materials are also capable of reversibly absorbing and desorbing Li ions. However, in the case of graphite, for example, the theoretical upper limit for the amount of Li to be absorbed is the amount required for formation of C6Li, that is, one Li atom per six carbon atoms, and the charge/discharge capacity thereof is 372 mAh/g.
Therefore, with the aim of achieving a further increase in the capacity of the non-aqueous electrolyte secondary battery, many studies have been undertaken on negative electrode materials. Among them is a proposal to improve carbon materials in order to achieve an increased battery capacity. For example, it has been reported that amorphous carbons and low crystalline carbons have a capacity much higher than the theoretical capacity of graphite (e.g., the 39th Battery Symposium Abstract volume, pp. 443-444 (3D12)).
Although amorphous carbons and low crystalline carbons have large theoretical capacity, they have the problem of having large irreversible capacity. The irreversible capacity refers to a capacity attributed to, of the Li ions absorbed in a carbon material, the Li ions which remain captured in the carbon material to be incapable of being desorbed during the subsequent discharge process and thus do not participate in the battery reaction any longer.
When a carbon material has irreversible capacity, a portion of Li ions which have been supplied to the carbon material in the negative electrode from a lithium-containing complex oxide in the positive electrode during the initial charge, is not able to return to the positive electrode during the subsequent discharge. Even in such case where a carbon material having large theoretical capacity is employed, it is difficult to obtain a high-capacity battery if the material has large irreversible capacity.
As a countermeasure against the irreversible capacity of a carbon material, an electrode formation process has been devised, by which lithium in an amount corresponding to the irreversible capacity is electrochemically charged into the carbon material in advance. The electrode formation process is excellent in that the formation can be controlled according to the amount of current to be passed. However, it necessitates charging an electrode once and using the electrode again to assemble a battery, resulting in complicated steps and low productivity.
As another countermeasure, a method of compensating for the irreversible capacity has been devised, which involves attaching metallic lithium to the negative electrode to automatically allow Li ions to move between the carbon material and the metallic lithium, which are in a state of short-circuit with the electrode interposed therebetween. In the case of this method, however, Li ions may not sufficiently move depending on the form of the electrode plate, so that metallic lithium remains in the negative electrode to cause variations in performance, safety problems and the like.
For the reasons as set forth above, little progress has been made in the practical use of amorphous carbons and low crystalline carbons, despite of the fact that they are promising as the negative electrode material.
Therefore, there is a demand for effective techniques that can be used in place of the ones described above in order to compensate for the irreversible capacity of carbon materials.